The present invention relates to light sources and in particular to a high frequency, low power light source employing light emitting diodes for illuminating an object photographed by a portable video camera.
Inspection systems sometimes employ a video camera to produce video signals representing light intensity at regularly spaced points on the surface of an object, for example a machine part to be engaged by an industrial robot. When a "frame grabber" converts the video signals into gray scale bit map data, a control computer can analyze the bit map data and determine the shape and position of the part. Some inspection systems use a variable frequency strobe light for illuminating a rotating or oscillating machine part monitored by the video camera to provide data representing the object as it appears at one or more selected positions during motion.
It is usually preferable that a light source uniformly illuminate a part under inspection. For example, if ambient light casts non-uniform shadows on the part, the camera can be fitted with a narrow band light filter for filtering out wavelengths of light other than the band of the light spectrum produced by the monochromatic light source. Since the intensity of ambient light "noise" within the narrow band is negligibly small compared with the intensity of the source, the ambient light interferes less with the monitoring process.
The light source should be light weight and rugged. It should require minimal maintenance, provide uniform illumination, generate little electrical noise or heat, and should not require a power source different from the one operating the camera. Incandescent, xenon, and fluorescent lights have been used as light sources, but have been found inconvenient or unacceptable in some applications because they require large, high voltage power supplies or generate too much heat or electrical noise. Such prior art light systems may also be too heavy or too fragile or may not be able to operate at a sufficiently high frequency or sufficiently low duty cycle.
Fiber optic ring lights provide a low infrared, uniform circle of light, but require a high intensity incandescent lamp as a source illuminator. The lamp is housed in a separate unit, and light is transmitted down a large diameter fiber optic cable to the ring light. The illuminator housings typically are very large in comparison to the ring light, require frequent replacement of the incandescent lamp, and generate large amounts of heat. In addition, the fiber optic cable coupling the lamp housing to the ring light is prone to internal fracturing due to the normal motions of a robotic end effector, requiring replacement. Fluorescent and cold cathode ring lights eliminate some of the above drawbacks in that they are light weight, generate minimal amounts of heat, and are uniform. However, problems inherent with these types of light sources are their requirement of a high voltage (&gt;200 V.) power supply, mechanical fragility and low light output.